1. Field of the Invention
The present invention relates to surface acoustic wave filters for use, for example, as band-pass filters. More specifically, the present invention relates to a surface acoustic wave filter including a plurality of one-terminal-pair surface acoustic wave resonators arranged in a ladder configuration, and also relates to a branching filter and a communication apparatus.
2. Description of the Related Art
In recent years, surface acoustic wave (SAW) filters including one-terminal-pair SAW resonators, which are used in a high-frequency band (RF band, in particular, GHz band or more) of communication apparatuses such as mobile phones, have been developed. The SAW filter is compact and lightweight, is highly resistant to shock and vibration, and is highly reliable with reduced variations. In addition, the circuit does not require adjustment, such that the filter can easily be automatically mounted. In addition, the filter is easily manufactured.
Although not shown, the one-terminal-pair SAW resonator includes a piezoelectric substrate, an interdigital transducer (hereinafter referred to as “IDT”) including an interdigital electrode (hereinafter referred to as “IDT electrode”) having a pair of comb electrodes, and two reflectors sandwiching the IDT on the right and left thereof (in the propagation direction of a surface acoustic wave (hereinafter referred to as SAW)), the IDT and the reflectors are aligned in the propagation direction of a SAW on the piezoelectric substrate.
The IDT is made of a metallic thin-film including aluminum or other suitable material, and functions as a SAW transducer, which transforms an input electric signal (AC) to a SAW (elastic energy) so as to propagate the SAW on the piezoelectric substrate and which also transforms the propagated SAW to an electric signal so as to output the signal. The reflectors reflect the propagated SAW in the propagated direction.
In such an IDT, a signal transform characteristic and a pass band are set by defining the length and width of each electrode finger of the IDT electrode, the pitch of adjacent electrode fingers, and the width of an overlapping portion of interdigitated electrode fingers. Also, in the reflector, a reflection characteristic is set by adjusting the width of each electrode finger and the pitch thereof.
As an example of the above-described SAW filter, Japanese Unexamined Patent Application Publication No. 5-183380 discloses a ladder band-pass filter in which a one-terminal-pair SAW resonator of a series arm and a one-terminal-pair SAW resonator of a parallel arm are alternately arranged.
In this ladder filter, a first one-terminal-pair SAW resonator is connected in series and a second one-terminal-pair SAW resonator is connected in parallel. By making the antiresonance frequency of the parallel resonator substantially correspond to the resonance frequency of the series resonator, very favorable filter characteristics of low loss and wide bandwidth are obtained. Therefore, this type of filter has been widely used in communication apparatuses.
Also, the above-described Japanese Unexamined Patent Application Publication No. 5-183380 discloses that a wide band filter characteristic is obtained by adding a series inductance to the series resonator or the parallel resonator.
However, this type of filter must have a favorable reflection characteristic and a steep attenuation characteristic at the vicinity of the pass band, in addition to a characteristic for use in a wide band. Also, in a filter typified by an Rx RF filter of GSM 1900 shown in FIG. 23, if attenuation bands exist in the vicinities of both sides of the pass band, adjustment is required so as to obtain a steep attenuation characteristic and an adequate pass bandwidth.
A method of applying a thinned electrode has been used to obtain a steep attenuation characteristic in the vicinity of the pass band. A thinned electrode is an IDT electrode in which a portion of electrode fingers is removed (see FIG. 24) or the polarity of a portion of electrode fingers is reversed (see FIG. 25) such that an electric field is not applied to the portion of the electrode fingers. Such thinning electrode is also referred to as withdrawal weighting.
Japanese Unexamined Patent Application Publication No. 11-163664 (published on Jun. 18, 1999) discloses a SAW filter including IDT electrodes which are uniformly thinned. In an embodiment thereof, a ladder filter is disclosed. As shown in FIG. 26, by thinning an IDT electrode, the antiresonance frequency is shifted toward the resonance frequency, such that the gap Δf between the resonance frequency and the antiresonance frequency is reduced. By applying this method to a ladder filter, adjustment can be performed, for example, the steepness in the vicinity of the pass band can be improved and the pass bandwidth can be reduced, as shown in FIG. 6 of the above-described Japanese Unexamined Patent Application Publication No. 11-163664.
Also, PCT Japanese Translation Patent Publication No. 2001-500697 (published on Jan. 16, 2001) discloses a ladder filter having similar advantages.
Further, Japanese Unexamined Patent Application Publication No. 9-153753 (published on Jun. 10, 1997) discloses the following method. In this method, weighting is performed such that the conductance of an IDT electrode is reduced to a small value in a desired frequency band. Further, by changing the electrode pitch, an adequate attenuation bandwidth is achieved. By using this method, a filter characteristic in which attenuation outside the pass band is steep and the attenuation bandwidth is wide is achieved. Incidentally, Japanese Unexamined Patent Application Publication No. 9-153753 does not disclose nor suggest thinning of the IDT electrode.
However, when a one-terminal-pair SAW resonator including a thinned IDT electrode is applied to a ladder filter, the attenuation characteristic of an attenuation band in the vicinity of the pass band increases, and thus, a frequency characteristic disadvantageously deteriorates.
The reason for this disadvantage is as follows. Due to a variation in the characteristic shown in FIG. 6 of Japanese Unexamined Patent Application Publication No. 11-163664, the impedance characteristic in the vicinity of a high-frequency side becomes less than the antiresonance frequency in a series resonator, and the impedance characteristic in the vicinity of a low-frequency side becomes greater than the resonance frequency in a parallel resonator. Accordingly, the impedance characteristic in the frequency range of the attenuation band varies.
FIG. 27 shows examples of the electrical frequency characteristic of a Tx filter, in which all series resonators are thinned at the same thinning ratio, which is varied, and the center frequency is adjusted by changing the entire electrode pitch so as to adjust the frequencies of the pass band and the attenuation band. The thinning ratio is a value indicating the ratio of thinned electrode fingers with respect to the number of pairs of electrode fingers of the IDT electrode.
FIG. 27 shows the above-described tendency: the frequency characteristic is deteriorated as the thinning ratio becomes higher. Therefore, even if a thinned electrode is applied to improve steepness, the attenuation characteristic of the entire attenuation band deteriorates due to a rise in the attenuation characteristic.
In the method described in Japanese Unexamined Patent Application Publication No. 9-153753, by weighting an electrode and changing the electrode pitch, the above-mentioned rise in the attenuation characteristic is suppressed and improved.
In Japanese Unexamined Patent Application Publication No. 9-153753, however, a thinned electrode is not disclosed and only an electrode using apodization is disclosed, which differs from the present application in its configuration. In the apodization, the area of an electrode is disadvantageously greater than that of a thinned electrode, and thus, a filter cannot be miniaturized as compared to the case where a thinned electrode is provided.
For example, if the electrodes of all series resonators are thinned at the same thinning ratio, the steepness is improved but the rise in the attenuation band is significant, as shown in FIG. 27.
On the other hand, if the electrode pitch is optimized, the rise in the attenuation band is reduced, but the reflection characteristic is deteriorated. Therefore, with the known configuration, it is difficult to optimize and improve both of reflection and attenuation characteristics.